Transparent thermoplastic materials, such as polycarbonates and polyacrylates, are used in a variety of substrates such as eyeglass, binocular, and camera lenses, as well as automobile, train, bus, and airplane windows. These thermoplastic materials are relatively light weight, tough, shatter resistant, and can withstand a wide range of temperatures and pressure gradients. One limitation, however, with these thermoplastic materials is that they are "relatively soft" and can be easily scratched and "crazed" or etched, especially under acidic conditions.
One particularly severe use of such thermoplastic materials is in aircraft transparencies, which, during flight, typically experience significant temperature variations (e.g., from -70.degree. F. to 110.degree. F.), severe pressure gradients as a result of cabin pressurization-depressurization cycles, relatively severe acidic environmental conditions, and severe weathering due mainly to ultraviolet light at high altitudes. As a result of such severe conditions, scratching and crazing of aircraft transparencies are accelerated, thereby inhibiting the optical clarity of the transparency.
In recent years, abrasion resistant coatings for transparent thermoplastic substrates (i.e., thermoplastic transparencies) have been developed that reduce the scratching and crazing of the substrates. See, for example, U.S. Pat. No. 3,488,215 (Shepherd et al.), U.S. Pat. No. 3,635,756 (Shepherd et al.), U.S. Pat. No. 4,885,332 (Bilkadi), U.S. Pat. No. 5,104,929 (Bilkadi), and U.S. Pat. No. 4,954,591 (Belinares). U.S. Pat. No. 4,954,591 discloses the use of N,N-(dialkyl)acrylamides in tintable abrasion resistant coatings for polycarbonate ophthalmic lenses. Tintability, however, as well as discoloration with aging, are not desirable properties for protective coatings designed for aircraft transparencies. In fact, the additives required to impart tintability typically severely degrade the abrasion resistance, the weatherability under ultraviolet light, and the acid resistance of radiation curable materials used in protective coatings.
It is believed that none of these coatings possess the properties necessary for protecting thermoplastic transparencies under the harsh conditions experienced by aircraft transparencies. Thus, a need exists for coatings that can withstand the severe conditions experienced by aircraft transparencies. Such coatings should ideally be resistant to temperature cycling, resistant to mechanical stresses, resistant to prolonged exposure to severe acidic conditions, and resistant to tinting and discoloration.